1. Technical Field
The invention relates generally to roller cone drill bits, and more specifically to simulating the drilling performance of roller cone bits. In particular, the invention relates to methods for generating a visual representation of a roller cone bit drilling earth formations, methods for designing roller cone bits, and methods for optimizing the drilling performance of a roller cone bit design.
2. Background Art
Roller cone rock bits and fixed cutter bits are commonly used in the oil and gas industry for drilling wells. FIG. 1 shows one example of a conventional drilling system drilling an earth formation. The drilling system includes a drilling rig 10 used to turn a drill string 12 which extends downward into a well bore 14. Connected to the end of the drill string 12 is roller cone-type drill bit 20, shown in further detail in FIG. 2. Roller cone bits 20 typically comprise a bit body 22 having an externally threaded connection at one end 24, and a plurality of roller cones 26 (usually three as shown) attached to the other end of the bit and able to rotate with respect to the bit body 22. Attached to the cones 26 of the bit 20 are a plurality of cutting elements 28 typically arranged in rows about the surface of the cones 26. The cutting elements 28 can be tungsten carbide inserts, polycrystalline diamond compacts, or milled steel teeth.
Significant expense is involved in the design and manufacture of drill bits. Therefore, having accurate models for simulating and analyzing the drilling characteristics of bits can greatly reduce the cost associated with manufacturing drill bits for testing and analysis purposes. For this reason, several models have been developed and employed for the analysis and design of fixed cutter bits. These fixed cutter simulation models have been particularly useful in that they have provided a means for analyzing the forces acting on the individual cutting elements on the bit, thereby leading to the design of, for example, force-balanced fixed cutter bits and designs having optimal spacing and placing of cutting elements on such bits. By analyzing forces on the individual cutting elements of a bit prior to making the bit, it is possible to avoid expensive trial and error designing of bit configurations that are effective and long lasting.
However, roller cone bits are more complex than fixed cutter bits in that cutting surfaces of the bit are disposed on the roller cones, wherein each roller cone independently rotates relative to the rotation of the bit body about axes oblique to the axis of the bit body. Additionally, the cutting elements of the roller cone bit deform the earth formation by a combination of compressive fracturing and shearing, whereas fixed cutter bits typically deform the earth formation substantially entirely by shearing. Therefore, accurately modeling the drilling performance of roller cone bits requires more complex models than for fixed cutter bits. Currently, no reliable roller cone bit models have been developed which take into consideration the location, orientation, size, height, and shape of each cutting element on the roller cone, and the interaction of each individual cutting element on the cones with earth formations during drilling.
Some researchers have developed a method for modeling roller cone cutter interaction with earth formations. See D. Ma et al, The Computer Simulation of the Interaction Between Roller Bit and Rock, paper no. 29922, Society of Petroleum Engineers, Richardson, Tex. (1995). However, such modeling has not yet been used in the roller cone bit design process to simulate the overall drilling performance of a roller cone bit, taking into consideration the equilibrium condition of forces and the collective drilling contribution of each individual cutting element drilling earth formations. The drilling contribution can be defined as the forming of craters due to pure cutting element interference and the brittle fracture of the formation.
There is a great need to simulate and optimize performance of roller cone bits drilling earth formations. Simulation of roller cone bits would enable analyzing the drilling characteristics of proposed bit designs and permit studying the effect of bit design parameter changes on the drilling characteristics of a bit. Such analysis and study would enable the optimization of roller cone drill bit designs to produce bits which exhibit desirable drilling characteristics and longevity. Similarly, the ability to simulate roller cone bit performance would enable studying the effects of altering the drilling parameters on the drilling performance of a given bit design. Such analysis would enable the optimization of drilling parameters for purposes of maximizing the drilling performance of a given bit.
In general, the invention comprises a method for simulating a roller cone bit drilling earth formations, which can be visually displayed and, alternatively, used to design roller cone drill bits or optimize drilling parameters for a selected roller cone bit drilling an earth formation.
In one aspect, the invention provides a method for generating a visual representation of a roller cone bit drilling earth formations. The method includes selecting bit design parameters, selecting drilling parameters, and selecting an earth formation to be drilled. The method further includes calculating, from the bit design parameters, drilling parameters and earth formation, parameters of a crater formed when one of a plurality of cutting elements contacts the earth formation. The method further includes calculating a bottomhole geometry, wherein the crater is removed from a bottomhole surface. The method also includes incrementally rotating the bit and repeating the calculating of crater parameters and bottomhole geometry based on calculated roller cone rotation speed and geometrical location with respect to rotation of said roller cone drill bit about its axis. The method also includes converting the crater and bottomhole geometry parameters into a visual representation.
In another aspect aspect, the invention provides a method for designing a roller cone drill bit. The method includes selecting initial bit design parameters, selecting drilling parameters, and selecting an earth formation to be drilled. The method further includes calculating, from the bit design parameters, drilling parameters and earth formation, parameters of a crater formed when one of a plurality of cutting elements contacts the earth formation. The method further includes calculating a bottomhole geometry, wherein the crater is removed from a bottomhole surface. The method also includes incrementally rotating the bit and repeating the calculating of crater parameters and bottomhole geometry based on calculated roller cone rotation speed and geometrical location with respect to rotation of said roller cone drill bit about its axis. The method further includes adjusting at least one of the bit design parameters and repeating the calculating until an optimal set of bit design parameters is obtained. Bit design parameters that can be optimized include, but are not limited to, cutting element count, cutting element height, cutting element geometrical shape, cutting element spacing, cutting element location, cutting element orientation, cone axis offset, cone diameter profile, and bit diameter.
In another aspect, the invention provides a method for optimizing drilling parameters for a roller cone drill bit. The method includes selecting bit design parameters, selecting initial drilling parameters, and selecting an earth formation to be drilled. The method further includes calculating, from the bit design parameters, drilling parameters and earth formation, parameters of a crater formed when one of a plurality of cutting elements contacts the earth formation. The method further includes calculating a bottomhole geometry, wherein the crater is removed from a bottomhole surface. The method also includes incrementally rotating the bit and repeating the calculating of crater parameters and bottomhole geometry based on calculated roller cone rotation speed and geometrical location with respect to rotation of said roller cone drill bit about its axis. Additionally, the method includes adjusting at least one of the drilling parameters and repeating the calculating until an optimal set of drilling parameters is obtained. The drilling parameters which can be optimized using the invention include, but are not limited to weight on bit and rotational speed of bit.